Optical transmission is a key requirement in many existing systems. There is often a need for transmission of optical signals or energy through free space. Such systems cover a wide range of applications in many fields related to data communications, metrology, energy delivery, sensor systems and medical instrumentation.
Key to the performance of such systems is the ability to propagate the optical energy through the medium in which the system operates. For many systems the medium consists of the atmosphere for a free-space communication system. Several properties of the atmosphere conspire to limit optical transmission, including atmospheric absorption, scattering (together known as extinction) and turbulence. In particular, clouds, fog, rain, dust, aerosols and smoke are especially detrimental to optical transmission.
Certain communications technologies in development rely on ultra-low low intensity beams, approximating single photon transmission, in which case extinction may be especially serious. These technologies may benefit from methods of transmission that reduce propagation loss.
Depending on the strength of extinction and turbulence, the strength of the optical beam may be attenuated, and other characteristics, such as direction and beam size, may also be adversely affected. In conventional attempts to overcome the limiting factors, a system can be built using more power (i.e., more energy projection) or made larger (e.g., with a larger aperture detector to recover more of a spreading beam).
Other methods may be employed to minimize or eliminate some of the problems. For example, in spatial diversity, multiple transmitters or receivers may be used to enhance transmission probability. Another solution for free space communications consists of a so-called hybrid system that employs a combination of both optical and either radio frequency (RF) or microwave (MW) beams (e.g., wavelength diversity). In wavelength diversity, a RF/MW frequency is used when necessary to transmit through a degraded optical channel. This method works because electromagnetic waves are most efficiently scattered by particles approximately the size of the wavelength. Hence longer wavelengths such as RF are not as affected by smaller particle obscurants such as aerosol or water droplets. However, in general, RF/MW data transmission rates are much lower due to the lower frequencies and require larger apertures to capture the more highly dispersive beams.
In any of these cases, the solution requires more equipment, complexity, weight, and size and results in higher cost of the overall system, simply because of duplication in many components.
As a result, there is a need for systems and methods for creating a low loss optical channel for free space communications that is compact, and low in cost and energy requirements.